The Trace Gas Orbiter has completed a 308 million-mile journey and now orbits the planet.
Photograph: ESA//PA

Scientists have begun an experiment aimed at solving one of astronomy’s most intriguing puzzles: the great Martian methane mystery.

In the next few months they hope to determine whether tantalising whiffs of the gas that have been detected on the red planet in recent years are geological in origin – or are produced by living organisms.

On Earth, methane is produced mostly by microbes, although the gas can also be generated by relatively simple geological processes underground. The ExoMars Trace Gas Orbiter, which has been manoeuvring itself above Mars for more than a year, has been designed to determine which of these sources is responsible for the planet’s methane. Last week sensors on the craft were deployed and began making their first measurements of the planet’s atmosphere.

“If we find traces of methane that are mixed with more complex organic molecules, it will be a strong sign that methane on Mars has a biological source and that it is being produced – or was once produced – by living organisms,” said Mark McCaughrean, senior adviser for science and exploration at the European Space Agency.

“However, if we find it is mixed with gases such as sulphur dioxide, that will suggest its source is geological, not biological. In addition, methane made biologically tends to contain lighter isotopes of the element carbon than methane that is made geologically.”

The ExoMars Trace Gas Orbiter was blasted towards Mars on a Proton rocket from the Baikonur cosmodrome in Kazakhstan in March 2016. The robot spacecraft – a joint European-Russian mission – reached its target seven months later and released a small lander, called Schiaparelli, which was designed to test heat shields and parachutes in preparation for future landings. However, the lander was destroyed when it crashed after its retro-thruster rockets shut off too early.

At the same time, the main orbiter swept into a highly elliptical path around Mars as planned.

Space engineers have since been altering that orbit – by repeatedly skimming the Martian atmosphere – so that the craft now circles the planet about 250 miles above the surface. A few days ago engineers pointed its instruments towards the planet and began taking measurements.

Scientists expect it will take more than a year to complete a full survey of the planet’s methane hotspots but are hopeful that within a month or two they will have a good idea if its source is biological or geological in origin.

Astronomers have found hints of methane on Mars on several previous occasions. In 2004, Europe’s Mars Express orbiter detected levels of methane in the atmosphere at about 10 parts in a billion. Ten years later, Nasa’s Curiosity rover recorded the presence of the gas on the surface. Crucially, atmospheric methane breaks up quickly in the presence of ultraviolet solar radiation. Its continued presence on Mars therefore suggests it is being replenished from a source somewhere on the planet.

“We will look at sunlight as it passes through the Martian atmosphere and study how it is absorbed by methane molecules there,” said Håkan Svedhem, the orbiter’s project scientist. “We should be able to detect the presence of the gas to an accuracy of one molecule in every 10 billion molecules.”

If the methane is found to be biological in origin, two scenarios will have to be considered: either long-extinct microbes, which disappeared millions of years ago, have left the methane to seep slowly to the surface – or some very resistant methane-producing organisms still survive underground. “Life could still be clinging on under the Martian surface,” said Svedhem.

However, if the gas is found to be geological in origin, the discovery could still have important implications. On Earth, methane is produced – geologically – by a process known as “serpentinisation” which occurs when olivine, a mineral present on Mars, reacts with water.

“If we do find that methane is produced by geochemical processes on Mars, that will at least indicate that there must be liquid water beneath the planet’s surface – and given that water is crucial to life as we know it, that would be good news for those of us hoping to find living organisms on Mars one day,” said McCaughrean.